Any of a group of genetic elements consisting of DNA and capable of giving selective advantage to the bacteria in which they occur. Episomes may be attached to the bacterial cell membrane or become part of the chromosome. Cells with episomes act like males during conjugation, a mating process in certain bacteria. During conjugation, cells lacking the episome may receive either the episome or the episome plus the genes to which it is attached. Experiments involving gene transfers from cells in which episomes have been incorporated in the chromosomes have been used to determine the locations of genes on the chromosome.